Damage Mechanism Of Gold-plated Grating In Ultra-short Pulse Laser System

Femtosecond laser ablation of gold-plated grating(ACG)shows damage such as thermal melting and thermal stress.Since the ablation mechanism involves light field modulation,thermal effects,field effects,film damage,etc.,it is still the focus of research in this field.In this paper,based on the optimized two temperature model,the dynamic process of gold-plated grating irradiated by femtosecond laser is studied,and the ablation damage mechanism of gold-plated grating is analyzed,which has a certain theoretical reference value for further improving its damage threshold(LIDT)and improving the performance of short-pulse high-power laser systems.The specific content and conclusions are as follows:(1)Define the absorption rate related to light field modulation,adopt dynamic thermophysical parameters and mechanical parameters,introduces grating groove-dependent depth and use Fower-Du Bridge(FD)electron emission theory as boundary conditions to optimize the classic dual-temperature model(TTM),use the grating elements and laser parameters in the experiment,and calculate the temperature and stress distribution and evolution of the upper and lower surfaces of the gold film are described.The results show that the thermal fusion damage threshold of the upper surface of the gold film is 0.54 J/cm~2,which is in good agreement with the experimental results.Regardless of the energy loss caused by electron emission,the thermal fusion damage threshold will be underestimated by 31.5%.During the irradiation process,the maximum value of the surface stress of the gold film can reach 0.3 GPa,which is close to the adhesion strength of the gold film.The maximum value of residual stress is located at the top of the ridge.For the gold-plated grating with low adhesion strength(about 0.321 GPa),the initial damage is manifested as thermal stress mechanical damage such as bubble peeling,which is in good agreement with the experimental results.In addition,electron emission has almost no effect on the surface stress under the gold film.(2)Define the absorption rate adjustment coefficient related to the groove structure,and study the influence of the grating structure parameters(gold film thickness,groove depth,duty cycle)on the temperature field and the stress field.It is found that the thickness of the gold film has basically no effect on the thermal fusion damage threshold.In a certain range,the stress increases with the increase of the thickness of the gold film.When the film increases to a certain thickness,the stress is basically constant.It shows that under the same adhesion strength,the thicker the gold-plated grating is more likely to cause mechanical damage such as bubble peeling;The groove depth has a significant influence on the temperature and stress distribution.As the depth increases,the thermal fusion damage threshold decreases,and the lower surface stress decreases,it is indicated that under the same adhesion strength,the greater the depth of the groove is,the more easily it causes the initial damage of thermal melting,and the smaller the groove is,the more easily it causes the initial damage of thermal mechanical stress;The duty cycle has a great influence on the thermal damage threshold and a small influence on the stress.(3)Fix the grating parameters,study the influence of multi-pulse laser(number of pulses,interval time)on temperature field and stress field,It is found that the number of pulses causes the accumulation of heat on the electron temperature and lattice temperature.Increasing the number of pulses reduces the thermal melting damage threshold,while the stress remains basically unchanged;As the interval time increases,the cumulative effect on the lattice temperature weakens,and the interval time basically has no effect on the lower surface stress.The multi-pulse laser beam increases the possibility that the initial damage is thermal fusion damage.